2,205 research outputs found
Low-energy expansion formula for one-dimensional Fokker-Planck and Schr\"odinger equations with asymptotically periodic potentials
We consider one-dimensional Fokker-Planck and Schr\"odinger equations with a
potential which approaches a periodic function at spatial infinity. We extend
the low-energy expansion method, which was introduced in previous papers, to be
applicable to such asymptotically periodic cases. Using this method, we study
the low-energy behavior of the Green function.Comment: author-created, un-copyedited version of an article accepted for
publication in Journal of Physics A: Mathematical and Theoretica
A tight lower bound for an online hypercube packing problem and bounds for prices of anarchy of a related game
We prove a tight lower bound on the asymptotic performance ratio of
the bounded space online -hypercube bin packing problem, solving an open
question raised in 2005. In the classic -hypercube bin packing problem, we
are given a sequence of -dimensional hypercubes and we have an unlimited
number of bins, each of which is a -dimensional unit hypercube. The goal is
to pack (orthogonally) the given hypercubes into the minimum possible number of
bins, in such a way that no two hypercubes in the same bin overlap. The bounded
space online -hypercube bin packing problem is a variant of the
-hypercube bin packing problem, in which the hypercubes arrive online and
each one must be packed in an open bin without the knowledge of the next
hypercubes. Moreover, at each moment, only a constant number of open bins are
allowed (whenever a new bin is used, it is considered open, and it remains so
until it is considered closed, in which case, it is not allowed to accept new
hypercubes). Epstein and van Stee [SIAM J. Comput. 35 (2005), no. 2, 431-448]
showed that is and , and conjectured that
it is . We show that is in fact . To
obtain this result, we elaborate on some ideas presented by those authors, and
go one step further showing how to obtain better (offline) packings of certain
special instances for which one knows how many bins any bounded space algorithm
has to use. Our main contribution establishes the existence of such packings,
for large enough , using probabilistic arguments. Such packings also lead to
lower bounds for the prices of anarchy of the selfish -hypercube bin packing
game. We present a lower bound of for the pure price of
anarchy of this game, and we also give a lower bound of for
its strong price of anarchy
Mathematical Models and Exact Algorithms for the Colored Bin Packing Problem
This paper focuses on exact approaches for the Colored Bin Packing Problem
(CBPP), a generalization of the classical one-dimensional Bin Packing Problem
in which each item has, in addition to its length, a color, and no two items of
the same color can appear consecutively in the same bin. To simplify modeling,
we present a characterization of any feasible packing of this problem in a way
that does not depend on its ordering. Furthermore, we present four exact
algorithms for the CBPP. First, we propose a generalization of Val\'erio de
Carvalho's arc flow formulation for the CBPP using a graph with multiple
layers, each representing a color. Second, we present an improved arc flow
formulation that uses a more compact graph and has the same linear relaxation
bound as the first formulation. And finally, we design two exponential
set-partition models based on reductions to a generalized vehicle routing
problem, which are solved by a branch-cut-and-price algorithm through
VRPSolver. To compare the proposed algorithms, a varied benchmark set with 574
instances of the CBPP is presented. Results show that the best model, our
improved arc flow formulation, was able to solve over 62% of the proposed
instances to optimality, the largest of which with 500 items and 37 colors.
While being able to solve fewer instances in total, the set-partition models
exceeded their arc flow counterparts in instances with a very small number of
colors
Mechanism of atomic force microscopy imaging of three-dimensional hydration structures at a solid-liquid interface
Here we present both subnanometer imaging of three-dimensional (3D) hydration structures using atomic force microscopy (AFM) and molecular dynamics simulations of the calcite-water interface. In AFM, by scanning the 3D interfacial space in pure water and recording the force on the tip, a 3D force image can be produced, which can then be directly compared to the simulated 3D water density and forces on a model tip. Analyzing in depth the resemblance between experiment and simulation as a function of the tip-sample distance allowed us to clarify the contrast mechanism in the force images and the reason for their agreement with water density distributions. This work aims to form the theoretical basis for AFM imaging of hydration structures and enables its application to future studies on important interfacial processes at the molecular scale
Efeitos da temperatura de secagem do solo e extratores na solubilidade do manganês.
Temperatura de secagem do solo e das soluções extratoras EDTA, CuCl2 e MgCl2 na solubilidade do manganes
Zircon to monazite phase transition in CeVO4
X-ray diffraction and Raman-scattering measurements on cerium vanadate have
been performed up to 12 and 16 GPa, respectively. Experiments reveal that at
5.3 GPa the onset of a pressure-induced irreversible phase transition from the
zircon to the monazite structure. Beyond this pressure, diffraction peaks and
Raman-active modes of the monazite phase are measured. The zircon to monazite
transition in CeVO4 is distinctive among the other rare-earth orthovanadates.
We also observed softening of external translational Eg and internal B2g
bending modes. We attributed it to mechanical instabilities of zircon phase
against the pressure-induced distortion. We additionally report
lattice-dynamical and total-energy calculations which are in agreement with the
experimental results. Finally, the effect of non-hydrostatic stresses on the
structural sequence is studied and the equations of state of different phases
are reported.Comment: 45 pages, 8 figures, 8 table
Magnetic and Metal-Insulator Transitions through Bandwidth Control in Two-Dimensional Hubbard Models with Nearest and Next-Nearest Neighbor Transfers
Numerical studies on Mott transitions caused by the control of the ratio
between bandwidth and electron-electron interaction () are reported. By
using the recently proposed path-integral renormalization group(PIRG)
algorithm, physical properties near the transitions in the ground state of
two-dimensional half-filled models with the nearest and the next-nearest
neighbor transfers ( and , respectively) are studied as a prototype of
geometrically frustrated system. The nature of the bandwidth-control
transitions shows sharp contrast with that of the filling-control transitions:
First, the metal-insulator and magnetic transitions are separated each other
and the metal-insulator (MI) transition occurs at smaller , although the
both transition interactions increase with increasing . Both
transitions do not contradict the first-order transitions for smaller
while the MI transitions become continuous type accompanied by emergence of
{\it unusual metallic phase} near the transition for large . A
nonmagnetic insulator phase is stabilized between MI and AF transitions. The
region of the nonmagnetic insulator becomes wider with increasing . The
phase diagram naturally connects two qualitatively different limits, namely the
Hartree-Fock results at small and speculations in the strong coupling
Heisenberg limit.Comment: 30 pages including 20 figure
Effect of magnetic frustration on single-hole spectral function in the t-t'-t''-J model
We examine the effect of the magnetic frustration J' on the single-hole
spectral function in the t-t'-t''-J model. At zero temperature, the exact
diagonalization (ED) and the self-consistent Born approximation (SCBA) methods
are used. We find that the frustration suppresses the quasiparticle (QP) weight
at small momentum k, whereas the QP peak at k=(pi/2,pi/2) remains sharp. We
also show the temperature dependence of the single-hole spectral function by
using the ED method. It is found that the lineshapes at (pi/2,0) and
(pi/2,pi/2) show different temperature dependence. These findings are
consistent with the angle-resolved photoemission data on Sr2CuO2Cl2, and
indicate the importance of the magnetic frustration on the electronic states of
the insulating cuprates.Comment: 5 pages, 3 EPS figures, REVTeX, To be published in Phys. Rev. B, Vol.
59, Num. 3 (15 Jan. 1999
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